This article was published in the December 2013 (#46) edition of Woodturning Design.
INTRODUCTION: I've been doing a lot of sphere related stuff lately, so it's hardly surprising that this years ornament is a Sphere Ornament. What was surprising however, to me at least, was my wife's reaction. Ordinarily she isn't all that into my ornaments. She says the long finial ones are hard to hang on the tree (not that she does that at our house) and dismisses others as "woodturner's ornaments. But when I showed her one of the early sphere ornaments she enthusiastically said "Now THAT'S a Christmas Ornament.
As I write this it's the start of May, and I've been trying sphere ornaments variations (and perhaps the editor's patience) since December. They're all done more or less the same way. A spindle blank with a temporary joint joining two halves is mounted on the lathe and turned into a sphere using the Shadow Sphere Jig (see WTD #39 or http://www.davidreedsmith.com/Articles/ShadowSphereJig/ShadowSphereJig.htm). The temporary joint is split and the hemispheres are quickly hollowed using a vacuum/tape/jam chuck (see WTD #45) and a bowl gouge. The hemispheres are then glued back together, the ornament sanded using cup centers (see WTD #44) and a finial added.
I tried LOTS of various ways of disguising the joint, while decorating the ornament. You can look at a gallery on my web site. The one I chose for this ornament is segmented from plywood. Two layers of 3/4" plywood are glued up and cut into equilateral diamond staves (if you're not into the angles use 4 square staves). The staves are oriented so that you're always turning downhill, making it much easier. I first tried plywood with the plies oriented tangentially to the circumference, which yields a nice looking ornament but is much harder to turn.
PLYWOOD: I tried various kinds of plywood, starting with what I had or what was available locally. Well, I didn't try BC fir plywood which is notorious for voids. I did try Birch plywood from Home Depot which is a very thin layer of birch plywood over poplar core layers. I had a lot of trouble with it chunking out when turning. Perhaps the relatively thick poplar plies are peeled off continuously resulting in fractures. I tried some Sande plywood, also from Home Depot, even though I didn't expect much from it as I've found it to be quite weak in the past, but it turned more easily, and with less chunk-out than the Birch plywood. Then I picked up some Baltic Birch plywood at the local Woodcraft. I thought this turned the best and looked nicest It has perhaps a more pronounced stripe effect from the plies, turned without chunk out, but seemed to dull the tools quickly. I recommend trying the Baltic Birch plywood, or if you want to procure locally and cheaply, the Sande plywood.
MAKING THE STAVES: The sphere ornament looks good about 3" or a little less in diameter, so the material to cut the staves out of needs to be about 1-1/2" thick. Extra room is needed at the ends for nubs so each stave should be about 4" long. To allow for cutting errors the staves are glued up to make half blanks and flattened. It's easier to flatten a longer piece, so cut two pieces of ~3/4" plywood 8" wide. I've found that polyurethane glue works most dependably for laminating. Apply sufficient glue to one of the plywood pieces and spread the glue evenly. I used a piece of cereal box cardboard as in Fig01. Clamp the two pieces together with lots of clamps as in Fig02 and let the glue cure over night.
To cut accurate bevels the edges of the plywood need to be straight and parallel. Using a jointer for the first edge and table saw for the second edge (or table saw for both) trim the edges as in Fig03. Now cut the bevels of the staves (if you're not into bevels you can use four 90 degree staves instead of six 60 degree staves. It will just look a little different. I'll include photos for this alternative in the online version of this article). I used my diamond rip sled to cut the bevels (see http://www.davidreedsmith.com/Articles/TriangleDiamondRipSled/TriangleDiamondRipSled.htm) Once I had set the blade angle to a trifle over 30 degrees (it's easier to fatten a concave surface than a convex one because it doesn't rock) I taped a piece of cereal box cardboard to the back fence of the sled and made a pass through the saw so I could measure from the actual kerf. For the first cut align the edge of the laminated plywood with the kerf and clamp it against the back fence and make the first bevel cut as in Fig04. Flip the laminated plywood over and adjust the fence to get the maximum width of stave. Register the laminate against the fence, and at least while the laminate was still long enough, clamp it against the back fence and cut the stave as in Fig05. Flip the board over and continue to cut until you have 3 staves. Of course you can cut more at once, but stop with three until you've glued up a set to confirm the settings are correct.
The staves will be glued together so that the plies radiate out from the center like a windmill. When turned in the proper direction, all the layers will have support from the layer behind, cutting down on tear out. First glue two of the staves together. I used regular titebond to glue the segments together, as I wasn't prepared to wait overnight twice...but if you've got the time it may very well be more dependable with polyurethane. If you think of the side which the plies are parallel to as the base, the base of one stave will be glued to the side of another. Look ahead to Fig09 to see the pattern. Spread glue on one of the joining surfaces. You can clamp across the points as in Fig06 with at least 3 clamps. Give the glue a chance to set, then glue the third stave to the other two. Be sure to continue the "windmill" orientation and clamp the joint as in Fig07.
After the glue has cured remove the clamps and flatten the surface with a jointer, hand plane or sander. Then cut into two blanks on the table saw as in Fig08, or with a bandsaw. So that you don't make mistakes in the hurry of gluing up, orient the half blanks so that you have a consistent windmill pattern and put some pencil marks across the edges as in Fig09.
TEMPORARY GLUE JOINT: The half blanks will now be assembled with a temporary glue joint for turning into a sphere. While you can use double stick tape or a paper joint, I recommend using blue tape because it doesn't require sanding off for clean up as Kraft paper does. Really, it's stronger than you think. Tape seems weak, because you're only breaking the bond a little bit at a time when peeling it off. When the whole surface is engaged it's really quite strong. To try and demonstrate this I used blue tape and Titebond glue to temporarily glue two pieces of 2x4 together, calculating to have the same joint area as a 3" sphere. I clamped one end to my basement ceiling joist. It held all 185 pounds of me, as you can see in Fig10. I repeated the experiment with a Kraft paper joint and it failed. I did find that I sometimes dented the hemispheres when splitting them apart using just blue tape and glue. So I added a layer of cereal box cardboard. This gave a marginally weaker joint (about the same as Kraft paper) and thick enough I could center my putty knife on the cardboard and pretty much confine any denting to the cardboard.
Cover the mating surfaces of the half-blanks with blue tape. Lightly sand the blue tape to remove the coating. Cut a piece of cereal box cardboard to size and sand the printed surface to remove at least some of the coating. Fig11 shows the set-up for gluing the temporary joint. Put a bead of glue on both blue taped surfaces. You can use the cardboard piece to spread the glue evenly. Place the cardboard on one half-blank, then align the other half-blank, paying attention to the alignment marks, and clamp the half-blanks together with at least two clamps.
TURNING THE BLANK: Give the glue a chance to cure and then remove the clamps. It will be easier to dimple and mark the blank if it has true ends, so clean up the ends with a pass on your table saw. Make a dimple at the intersection of the pattern with an awl or other pointed implement as in Fig 13, to aid in mounting the blank centered. Mount a cup drive center and cup tailstock center on your lathe. Cup centers are safer to use because they mechanically hold all the staves. Then mount the blank on the lathe. Be sure to mount the blank so that the plies will support one another when turning as in the left image of Fig14 (with a facet of the blank facing the tool rest, the plies should be angled up as they near the tool rest).
Turn the blank to a cylinder using a spindle roughing gouge, or whatever tool you prefer, as in Fig15. Then set up the shadow sphere jig (see WTD #39 or http://www.davidreedsmith.com/Articles/ShadowSphereJig/ShadowSphereJig.htm) as in Fig16. You'll note that since the article I've added a mirror behind the cylinder so that I can more easily see the unobstructed shadow of the back of the turning. It's a polished piece of sheet metal stuck in a wood strip that has magnets on the bottom. If you prefer a different method of turning a sphere go ahead and use it.
Using a spindle gouge (or whatever appeals to you), cut a bevel on the tailstock end of the cylinder until the shadow of the bevel is tangent to the circle pattern as in Fig17. Then round over the bevel with the spindle gouge until the shadow on the tailstock end matches the circle pattern as in Fig18, leaving a moderate nub. Repeat on the headstock end.
Now reduce the nubs with a skew, or a detail spindle gouge if you prefer, as in Fig19. Try for something small enough you can cut through with a utility knife. If you prefer you can leave a bigger nub, the hold the sphere crosswise with cup centers and saw off the nub. Cut off the nubs. Put alignment marks with a pencil somewhere on the temporary joint, as in Fig20, for alignment help when you glue it back together.
PREP FOR HOLLOWING: Now split the temporary joint. You may wish to make a starter groove with a utility knife in the middle of the cardboard at the alignment marks. Place the sphere on a folded rag to pad it a bit. Then center a wide putty knife (I sharpened mine a bit) on the joint and tap with a mallet to split the halves of the sphere as in Fig21. Measure the height of BOTH hemispheres with calipers (to avoid embarrassing holes in your ornament) as in the left image of Fig22. Lock the calipers at the smaller of the two measurements. Select a small drill (about 3/16") and a pair of locking pliers. Lock the drill in the pliers so that the drill point is the desired wall thickness from base of the calipers as in the right image of Fig22.
HOLLOWING THE HEMISPHERES: Mount the Sphere Vacuum/Jam/Tape chuck on your lathe. I'm using one just like in WTD#45 (or see my web site), except it's not eccentric capable--just a wooden faceplate with a 2mm craft foam lined 20 degree inside slope. Press one of the hemispheres into the chuck. If you have trouble getting it lined up with the flat surface straight across try pushing it in with the tailstock ram after removing the tailstock center. If you don't have a vacuum system (or if you just like insurance) wind some blue tape around the hemisphere and end of the chuck, then press it in place as in Fig23. Really, it will work, I've done it without vacuum many times. With a 20 degree inside slope the chuck is really a tape or vacuum assisted jam chuck. The jam does most of the heavy lifting--the tape just holds it in.
Set up your tool rest so that it extends more than all the way across the hemisphere, as that way if the hemisphere does come loose it will hit the tool rest and not your hand. Turn on the lathe and use a skew or other pointed tool to make a small starting dimple. Then lower the tool rest and use the drill with locking plier depth stop to drill a central hole as in Fig24. Then use a small bowl gouge to hollow the hemisphere as in Fig25. You must not have a catch when cutting at the rim. If necessary use a parting tool or skew as a scraper to form a starting ledge. I've found that the depth stop hole is good enough as a guide when hollowing to lighten the ornament--just hollow until the hole disappears. If you were using this technique to create a thin walled ornament suitable for piercing and need thin and uniform walls you would probably also need a template or other guide. Remove the hemisphere and mount and hollow the other one.
If you've used a paper joint clean up the rims now. Then spread some glue on one of the rims, bring the hemispheres together while aligning the registration marks and clamp together as in Fig26. Allow the glue to cure thoroughly before sanding.
SANDING: Assuming you're one of us mere mortals when it comes to tool control, you may wish to make a touch up paddle before starting to sand the sphere. A sanding disc, or any other power sanding device tends to leave a flat spot on the sphere. When you go back to hand sanding the abrasive tends to skip over the flat spot. A foam padded paddle, as in Fig27, however, when used with a rocking motion, doesn't create a flat spot. Yes, it won't be a perfect sphere anymore but we're making Christmas ornaments, not ball bearings. To make the paddle rip about 3/8" off the side of a 2x4 and cut it about 9" long. Cut two pieces of 2mm craft foam 1-1/2" wide and 5" long and glue to the face of the paddle with 3M #77 spray adhesive starting at one end. Cover the foam with duct tape. Then use 3M #77 spray adhesive to attach a strip of whatever abrasive you start sanding with.
Mount foam lined cup centers on your lathe as in Fig28. You could use the eccentric cup centers from WTD #44. The one's I'm using are much the same, except not eccentric capable. Mount the sphere between the cup centers by advancing the tailstock. Turn the lathe on at a moderate speed--as there may be some error in mounting the sphere, you want the speed low enough that the abrasive can follow the surface without skipping. Start with an abrasive grit that will allow you to sand out any defects before you die of old age. Then loosen the cups and rotate the sphere 90 degrees, as in Fig29 and sand the surface originally hidden by the cups. Change to the next finer abrasive and sand. Then rotate the sphere 90 degrees to sand the area covered by the cups with the second grit. Repeat until you've sanded to the final grit, and then remove the sphere.
FINIAL AND FINAL: I tried the ornament with just a hanger. It didn't look right without a small finial. To make a finial mount a piece of 3/8" dowel in a collet chuck or 4-jawed chuck with #1 jaws with a little more than 1" sticking out (the rest can hide inside the headstock for the next finial). With a skew create a starting dimple for drilling. Mount the size drill appropriate for the hanger you'll use in a pin chuck or in your tailstock mounted drill chuck and drill a hole for the hanger as in Fig30. Use a 1/4" skew or small spindle gouge to turn a ball on the end as in Fig31.
Select a parting tool and a gauge, or calipers set to the drill size you'll use to mount the finial. I used a #5 drill because that's the size gauge I had handy. Anything a little larger than 3/16" will do. Make a tenon to mount the finial with a parting tool cut sized with the gauge about 3/8" behind the ball as in Fig32. Then use a small spindle gouge to create a half cove that blends in with the parting tool cut as in Fig33. Then sand the finial with progressively finer grits and part it off the lathe with a narrow parting tool.
The set up to mount the finial is shown in Fig34. Create a drill starting dimple where the staves intersect with an awl and then drill a hole that matches your finial tenon. Put some glue in the drilled hole (if you don't mind a little awl abuse, put a drop of glue just behind the point of the awl and spread it around the sides of the hole) and insert the finial. Then apply finish. I used spray lacquer.
Fig35 shows the completed ornament. Fig36 shows a completed ornament using 4 square staves. I did lots of other variations. You can see some of them at www.DavidReedSmith.com/Gallery/SphereOrnaments/SphereOrnaments.htm.
Fig35: The completed ornament.
Fig35: The completed ornament.
Fig36: An ornament using 4 square staves.
Fig36: An ornament using 4 square staves.
3/4" (nominal) plywood
Blue masking tape (original)
Cereal box cardboard
Finish of choice
Lots of clamps
Jointer and/or Table saw
Spindle roughing gouge
Shadow Sphere jig
Small spindle gouge and/or skew
AUTHOR: As of this writing, in May of 2013, David Reed Smith lives in Hampstead, Maryland, with his not quite effective yet Medicare Card proudly displayed on the refrigerator. He welcomes comments and questions via email at david@DavidReedSmith.com. This article, along with about 60 others are available on his web site at www.DavidReedSmith.com.